Lighting devices with lighting emitting diodes (LEDs) or organic lighting emitting diodes (OLEDs) exhibit a high brightness and become more important nowadays in the general illumination market. These lighting devices are already employed today for signalling and signage due to their monochrome light. They are expected to replace conventional lamps like incandescent bulbs in the near future. Further advantages of LED lighting devices are their good reliability and their insensibility against vibrations. As a result of this, such lighting devices will especially replace conventional lamps in automotive applications for instance as a tail light.
The above mentioned lighting devices need significant electrical power. A red LED of a tail light requires for example 3W. However, the reliability of lighting devices with such LEDs and their relative light output depend very much on the junction temperature. An increased junction temperature through the high power consumption abbreviates the service life and reduces the relative light output. To limit these effects a good cooling of the lighting device is essential. Therefore, a lighting device may comprise a slug for establishing a thermal contact of high thermal conductivity and the slug is mounted on a heat sink.
In a car environment the minus potential of the battery and the chassis itself are normally connected to ground. Further the heat sink is also connected to ground. When using conventional driving circuits for the LED lighting device, the heat sink and the lighting device have to be electrically insulated against each other. This yields to an increased thermal resistance, which limits the relative light output.
The patent application US 2005/0068776 A1 describes a LED device mounted on a heat sink with a thermal resistance. It is necessary, however, to isolate both the outgoing leads of the LED device which are connected to an applied power supply. The LED device is directly mechanically and thermodynamically connected to a heat sink by a screw. Therefore a thermal resistance exists and furthermore it limits the performance of such a lighting device.
Therefore, it is an object of the present application to provide for sufficient protection of the lighting device against damages through overheating. It is a further object of the present patent application to provide for a long reliability of the lighting device and for a well and consistent radiation. Another object of the present application is to provide for a lighting circuit which allows an optimal cooling of the lighting device.